The coronaviruses are members of a family of enveloped viruses that replicate in the cytoplasm of animal host cells, and that are most commonly implicated with the common cold (B. N. Fields, D. M. Knipe, P. M. Howley, D. E. Griffin, Fields Virology (Lippincott Williams & Wilkins, Philadelphia, ed. 4, 2001); Holmes, K. V. 2001. Coronaviruses. In Fields' virology. D. Knipe, et al., editors. Lippincott Williams & Wilkins. Philadelphia, Pa., USA. 1187-1203). They are distinguished by the presence of a single-stranded plus-sense RNA genome about 30 kb in length that has a 5′ cap structure and 3′ polyadenylation tract. Upon infection of an appropriate host cell, the 5′-most open reading frame (ORF) of the viral genome is translated into a large polyprotein that is cleaved by viral-encoded proteases. Cleavage of this large polyprotein releases both structural and non-structural proteins. The coronavirus membrane contains three or four viral structural proteins. The membrane (M) glycoprotein is the most abundant structural protein; it spans the membrane bilayer three times, leaving a short NH2-terminal domain outside the virus (or exposed luminally in intracellular membranes) and a long COOH terminus (cytoplasmic domain) inside the virion. The spike protein (S) is a type I membrane glycoprotein that constitutes the peplomers. The small envelope protein (E) has been detected as a minor structural component in avian infectious bronchitis virus (IBV), transmissible gastroenteritis virus (TGEV), and mouse hepatitis virus (MHV) particles, but it has not been extensively characterized. Some coronaviruses also contain a hemagglutinin-esterase protein (HE). Coronaviruses attach to host cells through the spike (S) glycoprotein. The viral membrane proteins, including the major proteins S (Spike) and M (membrane), are inserted into the endoplasmic reticulum (ER) Golgi intermediate compartment while full length replicated RNA plus strands assemble with the N (nucleocapsid) protein. This RNA protein complex then associates with the M protein embedded in the membranes of the ER, and virus particles form as the nucleocapsid complex buds into the lumen of the ER. The virus then migrates through the Golgi complex and eventually exits the cell, likely by exocytosis (B. N. Fields, D. M. Knipe, P. M. Howley, D. E. Griffin, Fields Virology (Lippincott Williams & Wilkins, Philadelphia, ed. 4, 2001)). The site of viral attachment to the host cell resides within the S protein.
The coronavirus large polypeptide is also cleaved to release several nonstructural proteins, including an RNA-dependent RNA polymerase (Rep) and an adenosine triphosphatase (ATPase) helicase (Hel). These proteins, in turn, are responsible for replicating the viral genome as well as generating nested transcripts that are used in the synthesis of the viral proteins.
The coronaviruses include a large number of viruses that infect different animal species. The predominant diseases associated with these viruses are respiratory and enteric infections, although hepatic and neurological diseases also occur. Human coronaviruses identified in the 1960s (including the prototype viruses HCoV-OC43 and HCoV-229E) are responsible for up to 30% of respiratory infections (S. H. Myint, in The Coronaviridae, S. G. Siddell, Ed. (Plenum, New York, 1995), pp. 389-401. Marra M A, Jones S J, Astell C R, Holt R A, Brooks-Wilson A, Butterfield Y S, et al. The genome sequence of the SARS-associated coronavirus. Science 2003; 300:1399-1404). Coronaviruses are currently classified into three antigenic groups: group 1 and 2 include mammalian coronaviruses, and group 3 encompasses avian coronaviruses. Human coronaviruses associated with common cold-like diseases are included in both group 1 (CoV-229E) and 2 (CoV-OC43) (Siddell S. The coronaviridae. New York: Plenum Press; 1995).
A novel human coronavirus has been isolated from the oropharyngeal specimens of patients with severe acute respiratory syndrome (“SARS”), and termed SARS-associated coronavirus [SARS-CoV] (See; Peiris J, et al. Lancet 2003; 361:1319-25; and see Ksiazek T G, et al. N Engl J Med 2003; 348:1953). Experimental infection of macaques has confirmed that the SARS-CoV is the cause of SARS (See; Fouchier R A, et al. Nature 2003; 423:240; and see Kuiken T, et al. Lancet 2003; 362:263-70). Sequence analysis of the complete genome of SARS-CoV has shown an RNA molecule of about 29,750 bases in length, with a genome organization similar to that of other coronaviruses (Ruan Y J, et al. Lancet 2003; 361:1779-85; Rota P A, et al. Science 2003; 300:1394-9. Marra Mass., et al. Science 2003; 300:1399-1404). In spite of this similar organization, the SARS-CoV RNA sequence is only distantly related to that of previously characterized coronaviruses (Ruan Y J, et al. Lancet 2003; 361: 1779-85). Consequently, whether the SARS-CoV has “jumped” from a nonhuman host reservoir to humans and the molecular basis of such a jump remain unanswered questions (Cyranoski D, Abbott A. Nature 2003; 423:467). Some biologic features of the SARS-CoV described in vivo and in vitro differ from those of other coronaviruses previously identified. Among these features are the peculiar tropism of the virus for Vero cells (a continuous cell line established from monkey kidney epithelial cells), its capacity for growth at 37° C. (while other respiratory coronaviruses grow at lower temperatures), and its ability to infect lower respiratory tract tissues (Holmes K V. J Clin Invest 2003; 111:1605-9). These aspects render the molecular and biologic characterization of SARS-CoV important not only for understanding the determinants of its pathogenic potential but also for planning rational strategies of antiviral therapy and vaccination. Particularly important is the unique infectivity of SARS-CoV over other coronaviruses, which at least partially accounts for the severity of SARS in comparison to the common cold.